1. Field of the Invention
Embodiments disclosed herein relate to threads for tubulars. More specifically, embodiments relate to threads having a reduced clearance gap between portions of the pin thread and the box thread.
2. Background
Casing joints, liners, drill pipe, and drill collars (collectively referred to as “tubulars”) are often used in drilling, completing, and producing a well. Casing joints, for example, may be emplaced in a wellbore to stabilize a formation, to protect a formation against elevated wellbore pressures (e.g., wellbore pressures that exceed a formation pressure), and the like. Casing joints may be coupled in an end-to-end manner by threaded connections, welded connections, and other connections known in the art. The connections may be designed so as to form a seal between an interior of the coupled casing joints and an annular space formed between exterior walls of the casing joints and walls of the wellbore. The seal may be, for example, an elastomeric seal (e.g., an o-ring seal), a metal-to-metal seal formed proximate the connection, or similar seals known in the art. In some connections, seals are formed between the internal and external threads. Connections with this characteristic are said to have a “thread seal.” As used herein, a “thread seal” means that a seal is formed between at least a portion of the internal thread on the box member and the external thread on the pin member.
It will be understood that certain terms are used herein as they would be conventionally understood where tubular joints are being connected in a vertical position along a central axis of the tubular members such as when making up a pipe string for lowering into a well bore. Thus, the term “load flank” designates the side wall surface of a thread that faces away from the outer end of the respective pin or box member on which the thread is formed and supports the weight (i.e., tensile load) of the lower tubular member hanging in the well bore. The term “stab flank” designates the side wall surface of the thread that faces toward the outer end of the respective pin or box member and supports forces compressing the joints toward each other such as the weight of the upper tubular member during the initial makeup of the joint or such as a force applied to push a lower tubular member against the bottom of a bore hole (i.e., compressive force). The term “face” of the box is the end of the box member facing outward from the box threads and the term “nose” of the pin is the end of the pin member facing outward from the threads of the connection. Upon makeup of a connection the nose of the pin is stabbed into and past the face of the box.
One type of thread commonly used to form a thread seal is a wedge thread. In FIG. 1, a connection having a wedge thread is shown. “Wedge threads” are characterized by threads that increase in width (i.e., axial distance between load flanks 225 and 226 and stab flanks 232 and 231) in opposite directions on the pin member 101 and box member 102. Wedge threads are extensively disclosed in U.S. Pat. No. RE 30,647 issued to Blose, U.S. Pat. No. RE 34,467 issued to Reeves, U.S. Pat. No. 4,703,954 issued to Ortloff, and U.S. Pat. No. 5,454,605 issued to Mott, all assigned to the assignee of the present invention and incorporated herein by reference. On the pin member 101, the pin thread crest 222 is narrow towards the distal end of the pin member 101 while the box thread crest 291 is wide. Moving along the axis 105 (from right to left), the pin thread crest 222 widens while the box thread crest 291 narrows.
Generally, thread seals are difficult to achieve with free-running threads having broad crests and roots, however, the same thread forms may have thread seals when used for wedge threads. Various thread forms may be used for embodiments disclosed below. One example of a suitable thread form is a semi-dovetailed thread form disclosed in U.S. Pat. No. 5,360,239 issued to Klementich, and incorporated herein by reference. Another thread form includes a multi-faceted load flank or stab flank, as disclosed in U.S. Pat. No. 6,722,706 issued to Church, and incorporated herein by reference. An open thread form with a generally rectangular shape is disclosed in U.S. Pat. No. 6,578,880 issued to Watts. Each of the above thread forms are example thread forms that may be used for embodiments having either wedge threads or free running threads. Those having ordinary skill in the art will appreciate that the teachings contained herein are not limited to particular thread forms.
For wedge threads, a thread seal is accomplished by the contact pressure caused by interference over at least a portion of the connection between the pin load flank 226 and the box load flank 225 and between the pin stab flank 232 and the box stab flank 231, which occurs when the connection is made-up. Close proximity or interference between the roots 292, 221 and crests 222, 291, respectfully, completes the thread seal when it occurs over at least a portion of where the flank interference occurs. Higher pressure may be contained with increased interference between the roots and crests (“root/crest interference”) on the pin member 101 and the box member 102 and by increasing flank interference. This particular connection also includes a metal-to-metal seal that is accomplished by contact between corresponding sealing surfaces 103 and 104 located on the pin member 101 and box member 102, respectively.
A property of wedge threads, which typically do not have a positive stop torque shoulder on the connection, is that the make-up is “indeterminate,” and, as a result, the relative position of the pin member and box member varies more for a given torque range to be applied than connections having a positive stop torque shoulder. As used herein, “make-up” refers to threading a pin member and a box member together. “Selected make-up” refers to threading the pin member and the box member together with a desired amount of torque, or based on a relative position (axial or circumferential) of the pin member with the box member. For wedge threads that are designed to have both flank interference and root/crest interference at a selected make-up, both the flank interference and root/crest interference increase as the connection is made-up (i.e. increase in torque increases flank interference and root/crest interference). For wedge threads that are designed to have root/crest clearance, the clearance decreases as the connection is made-up. Regardless of the design of the wedge thread, corresponding flanks and corresponding roots and crests come closer to each other (i.e. clearance decreases or interference increases) during make-up. Indeterminate make-up allows for the flank interference and root/crest interference to be increased by increasing the torque on the connection. Thus, a wedge thread may be able to thread seal higher pressures of gas and/or liquid by designing the connection to have more flank interference and/or root/crest interference or by increasing the torque on the connection, however, this also increases stress on the connection during make-up, which could lead to failure during use.
Free-running threads used for oilfield tubular connections typically do not form thread seals when the connection is made-up. FIG. 2 shows a prior art connection having free-running threads. The free-running threads include load flanks 154 and 155, stab flanks 157 and 158, crests 159 and 162, and roots 160 and 161. As is typical of a connection with free-running threads, this connection relies on a positive stop torque shoulder formed by the contact of surfaces 151 and 152 disposed on the pin member 101 and the box member 102, respectively. The positive stop torque shoulder shown in FIG. 2 is commonly referred to as a “pin nose shoulder.” In other connections, the positive stop torque shoulder may instead be formed by the box face 163 and a mating shoulder (not shown) on the pin member 101. The positive stop torque shoulder may provide a seal. Unlike wedge threads, which make-up by the wedging of the pin thread and the box thread, free-running threads rely on the positive stop torque shoulder to load the connection during make-up. To make-up the connection shown in FIG. 2, the pin member 101 and the box member 102 are screwed together until the surfaces 151 and 152 are brought into abutment, at which point the pin load flank 154 and box load flank 155 are also in abutment. Additional torque is applied to the pin member 101 and the box member 102 to load the surfaces 151 and 152 and the pin load flank 154 and box load flank 155 until the desired amount of make-up torque has been applied to the connection.
The connection shown in FIG. 2 does not accomplish a thread seal because of the large gap 153 that exists between the pin stab flank 157 and box stab flank 158. The gap 153 occurs because of how free-running threads with positive stop torque shoulders are loaded. Applying torque to the connection during make-up against the positive stop torque shoulder causes the pin member 101 to be compressed while the box member 102 is stretched in tension. Note that when a box face shoulder is used, the box member 102 is compressed while the pin member 101 is stretched in tension. The force between the pin member 101 and the box member 102 is applied through the pin load flank 154 and box load flank 155. The pin stab flank 157 and the box stab flank 158 are not loaded during make-up. This results in contact pressure between the load flanks 154 and 155 and a gap between stab flanks 157 and 158. As discussed above, a wedge thread (as shown in FIG. 1) is able to form a thread seal in part because of the interference between the load flanks 225 and 226 and the stab flanks 232 and 231. For wedge threads, this occurs near the end of the make-up of the connection because of the varying width of the pin thread and the box thread. To have similar interference between the load flanks 154 and 155 and stab flanks 157 and 158 on a cylindrical (i.e. non-tapered) free-running thread, the interference would exist substantially throughout the make-up of the connection because the pin thread and the box thread have a continuous width. Further, root/crest interference, if any, would exist substantially throughout the make-up of the connection. This could lead to galling of the threads and difficulty in making up the connection.
The variance in thread width for a wedge thread occurs as a result of the load flanks having different leads than the stab flanks. A thread lead may be quantified in inches per revolution. Note that this is the inverse of a commonly used term “thread pitch,” which is commonly quantified as threads per inch. A graph of the leads for a prior art wedge thread is shown in FIG. 3A. For this connection, the load lead 14 is constant over the length of the connection and greater than the stab lead 12, which is also constant. The nominal lead is shown as item 10. As used herein, “nominal lead” refers to the average of the load lead 14 and the stab lead 12. The thread will widen with each revolution by the difference in the load lead 14 and the stab lead 12. The difference in the load lead 14 and the stab lead 12 is sometimes referred to as the “wedge ratio.” For a free-running thread (i.e. non-wedge thread), the load lead 14 and the stab lead 12 would be substantially equal causing the free-running thread to have a substantially constant thread width (i.e. a zero wedge ratio).
Generally, a thread is cut on a tubular using a substantially constant thread lead (including the load lead and the stab lead), however, some variance in the thread lead occurs during the manufacturing process, which typically includes machining with a mill or lathe. During machining, the variance in the thread lead manifests as a slight periodic variation in the thread lead above and below the intended value for the thread lead. This phenomenon is commonly referred to as “thread drunkenness.” The amount of thread drunkenness that occurs is largely dependent on the machine being used. It may be caused by slop or backlash in the machine tool that is cutting the thread. The material being machined and the dimensions of the part being machined are also variables that affect the amount of thread drunkenness. Thread drunkenness can also occur as a result of the electronic controls “hunting” the location for the machine tool. Typically, thread drunkenness is on the order of 0.00005 inch to 0.0005 inch from nominal and is not visible to the eye. The period of the thread drunkenness is typically at least once per thread turn. Greater than normal thread drunkenness is visible as “chatter” on the thread surface and may result in the connection being scrapped. Generally, manufacturers try to eliminate any variations from nominal, such as experienced with thread drunkenness.
Intentional variances in thread leads have been disclosed in the prior art for the purposes of load distribution, however, the present inventor is unaware of variances in thread leads to form a thread seal for a wedge thread or a free-running thread. One example of a varied thread lead for stress distribution is disclosed in U.S. Pat. No. 4,582,348 issued to Dearden, et al., incorporated herein by reference in its entirety. Dearden discloses a connection with free-running threads that has the pin thread and box thread divided into three portions with different leads (note that Dearden refers to thread pitch, which is quantified as threads per inch). In FIG. 3B, a graph of the thread leads for the box member and the pin member is shown. As shown in the graph, at one end of the connection, the pin thread lead 21 is larger than the box thread lead 22. In the intermediate portion 23, the pin thread lead 21 and box thread lead 22 are substantially equal. At the other end of the connection, the box thread lead 22 is larger than the pin thread lead 21. In Dearden, the changes in the pin thread lead 21 and box thread lead 22 are step changes (i.e. substantially instantaneous changes in the lead). The varied thread leads disclosed by Dearden are intended to distribute loading across a greater portion of the connection, and have no effect on the inability of the free-running threads to form a thread seal. Dearden does not disclose varying a load lead or stab lead independent of each other.
Another connection is disclosed in U.S. Pat. No. 6,976,711, entitled “Threaded Connection Especially for Radially Plastically Expandable Conduit” (“Sivley”), assigned to the assignee of the present invention, and incorporated herein by reference in its entirety. Sivley discloses connections having a variance in load lead and/or stab lead on one or both of the pin member and the box member. A graph of an embodiment disclosed by Sivley is shown in FIG. 3C. Sivley discloses varying the load lead 14 relative to the stab lead 12 at a selected rate over at least a portion of the pin thread and/or box thread. In FIG. 3C, the connection is a wedge thread as shown by the difference between the load lead 14 and the stab lead 12. The load lead 14 and the stab lead 12 converge at a linear rate towards the end of the thread. Sivley discloses various other embodiments having load leads 14 and stab leads 12 that vary at linear rates relative to each other. The variance in the thread leads distributes the loads experienced by the connection over the length of the connection.
In the prior art, free-running threads suitable for oilfield tubulars fail to provide thread seals suitable for the pressure differentials experienced by the tubulars in the downhole environment. Wedge threads provide thread seals, but have difficulty sealing gases, which are more difficult to seal than fluids. Also, any improvement in the thread seal is generally desirable. What is still needed is a thread seal for free-running threads and an improved thread seal for wedge threads.